Amazon offers the Best Price Mattress 12" Memory Foam King Mattress, model no. OLC-FMS-1200K, for $307.79 with free shipping. That's the lowest total price we could find by $16. It features a 2-1/2" memory foam layer, 3" super-soft foam layer with pressure relief system, and 6-1/2" high-density foam layer.
Also available is the Best Price Mattress 8" Tight Top iCoil Spring Queen Mattress with Metal Platform Bed Frame, model no. 8.42E+11, for $186.99 with free shipping when you clip the $20 off coupon on the product page. That's $76 off and the best deal we could find. Features include independent operating coils and a steel frame with 14" of under-bed storage.